What do people mean when they talk about filter in writing? Like "watch out for filters" i'm so confused D:

I honestly didn’t know what you meant by this either and looked it up. Apparently - 

“Filtering” is when you place a character between the detail you want to present and the reader.

- and - 

Common filter words seem to include -

to seeto hearto thinkto touchto wonderto realizeto watchto lookto seemto feel (or feel like)canto decideto sound (or sound like)

Here is a paragraph a blog presented with filter words intact so you can see an example - 

Maria noticed James had gone silent and was staring past her out the coffee shop window. Turning and looking herself, she saw a beautiful woman on the sidewalk, talking animatedly on a mobile phone. Maria felt a little tug of jealousy, and she turned back around furiously and stared at James, who jumped and looked down at his espresso. She realized he wasn’t blushing, but he seemed uncomfortable. “Who is that?” she asked as she watched his face for any sign of guilt. She wondered if he could ever tell her the truth. She decided right then this would be his last chance to regain her trust.

Here is the blog post I pulled from - 

http://www.invisibleinkediting.com/2013/12/23/how-to-find-filter-words-and-filter-them-out/

But there are plenty on the subject if you Google it, so I suggest you just read up on the subject to learn more. (Google “filter words writing”).